The present invention relates to a novel blast effect test accelerometer transducer and to a novel method of use thereof to ascertain among other things, instantaneous velocity of surfaces subjected to sudden acceleration and/or the amount of deflection of armor plate when subjected to explosive blast forces. Unlike any such prior art motion-sensing accelerometer transducers of which I am aware, this novel accelerometer device is capable of withstanding and measuring exceptionally high levels of structural motion, i.e., in the domain of from about 1,000,000 to 10,000,000 Gs. This enables measurement for the first known time of deformation of steel armor plate when subjected to high energy explosive blasts, such as land mines.
Scaled model tests have long been the basis for military tank floor studies because of both safety considerations and the complexity of blast-structure interaction phenomena. The objective for scale testing is to obtain quantitative data for prototype design, to identify significant design variables, to estimate the extreme performance potential of an existing prototype system, and to investigate new phenomena when there are no alternatives. It should be noted that reduced scale testing for metallic plate plastic deformation response can only be presumed to be an accurate representation of prototype plastic deformation response if the true full scale plastic failure criteria is invariant; however, such is not the case for all materials in general. Strain rate effects and related yield phenomena cannot be accurately scaled. The general design rule of thumb is that any such model tests are to be interpreted in terms of deformation, and that prototype tests be used to obtain failure criteria. Therefore, when testing relative to military tanks or similar armored vehicles whose floor armor is subjected to mine blasts or the like, the degree of plate displacement is the important measurement in such tests. Thus, in such tests, the corresponding displacement of a tank floor plate is deemed to be characterized by a step increase in force attendant an accompanying step increase in acceleration thereby producing useful data including a continuous time history of velocity and related time displacement.
Most types of prior art motion-sensing transducer accelerometers designed to sense velocity and displacement of which I am aware are not suitable for use in scaled dynamic test models of the type being discussed herein. Examples of known force and motion-sensing transducers are the quartz sensors or transducers manufactured by PCB Piezotronics, a company located in Buffalo, N.Y. 14225. Other piezoelectric and piezo resistant type accelerometers are available from a company named ENDEVCO. Some of the prior art's unsuitability is attributed to both an inability to withstand and measure the above-stated exceptionally high force levels (1,000,000-10,000,000 Gs) as by breakage of the quartz sensor leads or probes, and also the requirement that relatively large weight masses be attached to the test model therefore seriously affecting the inertial properties of the armor system being tested.
The novel testing method in essence utilizes an explosive charge customarily of generally disc geometry having diameter d and thickness a (FIG. 5) which is placed a given distance, R, from the bottom of the test plate. Upon detonation, a time expedant pressure is applied to the test plate resulting in a positive impulse, designated by the symbol I, which is bounded by time t. Using well known scaling relationship and the Hopkinson blast wave scaling law, in conjunction with one-quarter (1/4) scale tests, i.e., physical scaling factor .lambda.=1/4, the scaled model parameters are obtained. In this regard reference is made to the publications "Explosions In Air" by W. E. Baker, University of Texas Press, Austin, Tex., 1973; and also to D. J. Schuring's "Scale Models in Engineering," Pergamon Press, New York, 1977, and W. E. Baker et al. "Similarity Methods in Engineering Dynamics, Theory and Practice of Scale Modeling," Hayden, Rochelle Park, N.J., 1973. Although the use of the Hopkinson blast law at close standoff distance could lead to misleading results since the expansion and shock waves of the blast diffuse or decay at different rates which are not subject to scaling, sufficient experimental evidence exists to indicate that the law is a reasonable approximation of the events which occur during the explosion. In this regard, reference may be made to a publication authored by Wenzel, A. B. and Esparza, E. D., entitled "Measurements of Pressures and Impulses at Close Distances from Explosive Charges Buried and in the Air," Army Mobility Equipment Research and Development Center, TR SwRI02-31231, August 1972. Also, the time scale impulse and plate response must be interpreted in a homologous manner. It must be emphasized that the scaling used in this method assumes that viscosity, strain rate, gravity, and other nonlinear thermoviscoelastic phenomena can be neglected.
Scaled model response data exhibits wide scatter. For this reason, the fixity of the model test plate should approach classical clamped conditions; that is, the boundaries of the test plate should be relatively fixed in space so that both shear moments must be readily reacted by the holding fixture; and the holding fixture also should be representative of the full scale apparatus being tested. For example, in the instant case, the scale should approach the ratio exhibited by the tank vehicle to its belly armor, i.e., a 28:1 hold-down weight ratio of the test plate fixture to test plate. The overall length of the test fixture was 6 ft (183 cm), its width was 31 inches (78.7 cm); and the model tank floor plate measured 2 ft (61 cm) by 2 ft (61 cm), having an area 15" (38 cm) by 15" (38 cm) exposed to react to the blast load. When the displacement of the plate occurs, the surface of the plate is temporarily deformed and forced into contact with the pins of the accelerometer transducer, thereby generating transient plate deformation data.
The accelerometer transducer is comprised of at least two, and preferably of six to eight pure annealed copper pin-type probes which are housed in a rigid collar flange and body block on a support fixture so that the pins project downwardly as shown at different lengths. Each probe is operatively connected in an initially open circuit manner in series with a resistance capacitor and an oscilloscope or other suitable fast recording means. These plurality pin probe circuits are in parallel connected with an electrical power source via an "on-off" switch and also collectively with a negative ground point on said test plate. When the deformation of the plate occurs while being subjected to a blast force, each physical contact by a copper pin with the upper surface progressively closes or activates the respective circuits and thereby discharges its circuit's single channel discharge capacitor with the resulting signal being recorded on the oscilloscope or other recording means, and also preferably on a tape recorder. The recorder is used to record the acoustical equivalent of the electrical signal produced on the oscilloscope.
In this manner the electrically measured time of arrival of the test plate surface is obtained. Since the relative distance of the end of each of the pin probes from the test surface is measured prior to blast detonation (these distances being represented by .DELTA.x), the average velocity may be obtained by adding together the relative velocity as measured at each of the pins and dividing that number by the number of pin probes used in the test. The average velocity for each plate tested is obtained by dividing the known distances (.DELTA.x) between the ends of the pins and the plate surface by the time increments between contacts. The time from detonation ignition of the blast explosive to plate-to-pin contact is obtainable from photographic prints of signal sweep traces on the respective oscilloscopes. By subtracting the time between pin or probe contacts, the incremental times (designated .DELTA.t) are obtainable. The average velocity of the plate surface is then computable from ((.DELTA.x)/.DELTA.t) and thus assumed to occur at ((.DELTA.t)/2). In addition, instantaneous velocity measurements at the time of respective pin contact can be determined from engineering graphs to obtain the stress intensity-plastic deformations of a given rod relationship for different known rod materials, as the basis for a velocity measuring transducer. By assuming initial incompressible flow for the copper pin at the time of the test surface-to-pin probe contact, the instantaneous velocity at that time can be obtained by measuring the pin probe diameter at the end which contacts the test surface both before and after the test is concluded, and using known velocity longitudinal strain relationships. Thus, the time of arrival of the test surface to each pin probe enables the calculation of average acceleration of the test plate surface.